Trace element fractionation in water-bearing silicic magmas

Crystallization of a hydrous andesite within a thermal boundary layer at 500 MPa pressure is simulated experimentally using the intrinsic thermal gradient of 10 mm length capsules in a horizontally arranged piston-cylinder apparatus. Magma solidification is programmed at two distinct cooling rates, slow (0.6 °C/h) and rapid (9.6 °C/h). Bulk laser ablation (LA-ICP-MS) analyses across the thermal gradient shed light about fractionation efficiency for trace elements at conditions of slow cooling in which, water-rich fluids favours element mobility. Compositional and textural features of our experiments provide new insights on the kinetics of trace element fractionation in water-bearing intermediate magmas. These features, together with the unrealistic diffusion values measured in the capsules (close to 10 –6 cm 2 s −1 ), indicate that incompatible elements co-migrate with a water-rich fluid phase, expelled from a crystal-rich network or mush, by gas-driven filter pressing. In the studied system, diffusive transport proceeded in the same direction of chemical elements migration by advection. It is proposed that liquid segregation is particularly effective at the thermal boundary layers created at the margins of ascent conduits and the walls of magma chambers. Fluxing of a trace element-rich fluid into the hotter, crystal-poor areas, at central and/or upper zones of magma chambers, gives rise to compositional zoning and, eventually, to the formation of silicic cupolas, which are preferential places for ore deposit generation.